Estimates of chemical fluxes from Daikoku Seamount - a shallow, erupting, submarine volcano on the Mariana Arc.

Submarine magmatism is the dominant mechanism controlling the transfer of heat and chemicals from the Earth’s crust into the ocean. Hydrothermal circulation from submarine arc volcanoes are important contributors to the global geochemical budgets for many elements, but quantitative flux estimates from these systems are exiguous. During the Submarine Ring of Fire Ironman Expedition in November 2014 we sampled the water column around Daikoku Seamount, a shallow (325 m) submarine intraoceanic arc volcano located in the Northern Mariana Islands. We mapped a hydrothermal plume (~300 m) originating from Daikoku with physical and chemical plume characteristics consistent with an erupting volcano. Specifically, H₂ concentrations exceeded 1000 nM – a concentration diagnostic of the interaction of seawater with lava or extremely hot rock. We also observed an elevated optical backscatter signal accompanied by strong oxygen redox potential (ORP) anomalies; a lowered pH; and enriched concentrations of ³He, total dissolvable Mn (TDMn), TDFe, particulate Al (pAl), pSi and pS. Deep particle plumes (>600 m) found on the flanks on the volcano, below the eruptive plume, further support the presence of an eruption. Chemical flux estimates from Daikoku were made by combining shipboard hull-mounted Acoustic Doppler Current Profile data with continuous and discrete Conductivity, Temperature and Depth (CTD) data and analyzed water chemistry. Towed CTD sections were conducted perpendicular to the current direction – a sampling strategy that optimizes the estimation of chemical fluxes by reducing complexities introduced by temporal variability in the speed and direction of plume dispersion. The summit of Daikoku lies just beneath the oceanic mixed layer and could inject hydrothermal fluids (with appreciable quantities of Fe) into the euphotic zone, potentially impacting oceanic productivity in the region and beyond.